However, there had been almost 700 emergency hospital admissions for the condition – more than 43 times the number declared by IVF doctors. There has also been a huge increase in the number of women suffering with the OHSS, the Mail has found. At least four women in the UK have died as a result of it.

Last night, the Human Fertilisation and Embryology Authority said it was asking urgent questions of IVF clinics. Chairman Sally Cheshire told the Mail that it was ‘very concerned’ fertility doctors might be under-reporting cases of OHSS.

MPs called for an overhaul of legislation to protect women. The revelation comes on the third day of a major investigation by the Daily Mail into the fertility industry.

We have revealed how IVF clinics convince women on low incomes to donate eggs in return for cash or free treatment.

Clinics also give women false hope of delaying motherhood by exaggerating their success rates with frozen eggs. The HFEA has launched a full inquiry into the clinics involved.

Women who have IVF are at risk of developing OHSS because of the hormone injections they have to take to stimulate egg production can make their ovaries go into overdrive. It is only possible to develop the condition as a result of fertility treatment.

In severe cases, OHSS will cause the ovaries to expand to more than 12cm – about five times their usual size. Excess fluid can accumulate in the abdomen, or around the lungs and heart, causing severe pain. This can lead to circulation and kidney problems, with some women having to be treated in intensive care with intravenous infusions.

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Clinics are obliged to report cases of patients having to be hospitalised because of severe or critical OHSS. But experts say the term ‘severe’ is vague.

Last year, there were 60 cases of severe or critical OHSS reported by UK clinics. But the NHS recorded 865 cases of women with OHSS going to hospital – and 836 were so serious they were emergency admissions. In 2010, clinics reported 16 cases of OHSS to the HFEA. But that year there were 691 emergency admissions.

In total, IVF clinics have reported 256 cases between 2010 and 2015. But over this time, there have been 4,792 admissions, with 4,587 classed as emergencies.

Labour MP Siobhain McDonagh has raised the issue of OHSS in parliament

As IVF has become increasingly popular, cases have risen. In 1995/96, there were a relatively low 404 emergency OHSS admissions.

IVF expert Dr John Parsons, a retired consultant at King’s College Hospital, said: ‘The definition of “severe” is so vague. If the HFEA wants to get a true picture of OHSS it should insist that clinics report all cases when the patient has to be hospitalised.’

Siobhain McDonagh, who as a Labour MP sat on the Women and Equalities Select Committee and has raised the issue of OHSS in parliament, said: ‘The Mail has done a huge service by uncovering these figures. This is ground-breaking.

‘The difference between the NHS statistics and the figures reported are incredible. The law needs to specifically mention the need for doctors to protect the welfare of women.’

Tory MP Dr Sarah Wollaston, who was chairman of the health select committee, said: ‘We need to look carefully at the reasons for the discrepancies.’

The HFEA said that in some cases, women could have been admitted with mild or moderate OHSS, which might explain why the NHS data is higher.

'DRUGS MADE MY OVARIES SWELL TO FIVE TIMES THEIR USUAL SIZE'

Karen Roscoe-Bailey’s ovaries swelled to five times their normal size because of IVF drugs

Karen Roscoe-Bailey’s ovaries swelled to five times their normal size because of IVF drugs.

She was breathless and lethargic and her stomach became so bloated she appeared to be seven months pregnant.

The charity manager had to be rushed to hospital, where doctors discovered she had fluid in her lungs and abdomen.

Mrs Roscoe-Bailey, 44, from Pontefract, West Yorkshire, suffered with ovarian hyperstimulation syndrome six years ago, during her second of four rounds of IVF.

She and her husband Mark, 42, an IT director, were terrified and did not feel they had been properly warned about the condition by their consultant.

Mrs Roscoe-Bailey was put on bed rest for three weeks and she had her stomach measured every day while doctors tried to drain the fluid inside her. She later discovered she was pregnant – and was delighted – but the effects of OHSS were severe.

‘My stomach never really went back to normal,’ she told the Mail. ‘By the time the fluid was gone, I was about five months pregnant and had a big baby bump.

‘I didn’t see my baby grow, or experience my pregnancy in a normal way, which is something I was not prepared for. I felt really panicked because breathing was so difficult.’

After battling OHSS, Mrs Roscoe-Bailey gave birth to her daughter Georgia in 2012. She and her husband continued with IVF treatment and had four cycles in five years, spending about £20,000. However, it was unsuccessful.

During subsequent IVF cycles, Mrs Roscoe-Bailey often felt the symptoms of OHSS were on the verge of returning – but doctors monitored her drugs to make sure she did not get it again.

After stopping treatment, they naturally conceived their second daughter, Harlie, who was born eight months ago.

Mrs Roscoe-Bailey added: ‘I educated myself on the whole IVF industry and OHSS wasn’t something I was ever told about. There should be more information so women are aware of the risks. Thankfully, we have our girls.’

■Do you feel you were exploited by a fertility clinic? Contact the Daily Mail Investigations Unit via ivf@dailymail.co.uk